1. Field of the Invention
The present invention relates to a fixing device used on electrophotographic copying machines, printers and facsimile (transmitting and receiving) equipment, in particular, and to an improvement on such a fixing device for fixing toner images on recording media using a radiating heat source.
2. Description of the Related Art
An electrophotographic copying machine has a fixing device for fixing toner images transferred onto recording media such as a sheet of recording paper or transfer material. Various kinds of fixing devices are known. A typical thermal roller type fixing device comprises a fixing roller, which is sometimes called a heat roller, that thermally fuses the toner on the sheet, and a pressure roller, which is sometimes called a backup roller, that presses upon the fixing roller to nip and hold the sheet in place. The fixing roller is formed in the shape of hollow cylinder. A heat source is held on the longitudinal axis of this fixing roller by means of a holding apparatus.
A typical fixing roller found in a conventional thermal fixing device comprises a metallic body made of aluminum, iron or the like, the surface of which is covered with a releasing material such as silicon rubber and PTFE (polytetrafluoroethylene). It is sometimes coated with a certain type of oil such as silicon oil as needed. On the other hand, the pressure roller typically comprises a metallic core coated with heat-resistant rubber such as silicon rubber, the surface of which is covered with a plastic tube made of Teflon if need be.
The heat source comprises a tube-like heater such as a halogen lamp, which heats up when a predetermined voltage is supplied. Since this halogen lamp is on the longitudinal axis of the fixing roller, the inner wall of the fixing roller is irradiated evenly with the heat generated by the halogen lamp, and the distribution of temperature of the outer wall of the fixing roller becomes uniform as to the circumferential direction. Then, the outer wall of the fixing roller is heated up to a temperature suited for fixation (e.g., 150-200.degree. C.).
Under such a condition, the fixing roller and the pressure roller rotate in directions opposite to each other, nipping and holding the sheet that is attached with the toner. At an area of pressure between the fixing roller and the pressure roller (hereinafter called the "nipping area"), the toner on the sheet is fused by the heat from the fixing roller and is fixed on the sheet by the pressure applied between the two rollers. After the toner is fixed, the sheet is transferred by the paper discharge roller and discharged on an output tray.
In such a fixing device, the heat transfer to the fixing roller is based on the heat radiation from the halogen lamp built therein. In order to transmit the heat efficiently, the inner surface of the fixing roller is coated with a black paint of heat-resistant resin with a high thermal radiation to achieve the thermal radiation of the inner surface of the fixing roller higher than "0.9". This allows the fixing roller to be efficiently heated.
A fixing device equipped with a radiant heater consisting of a halogen lamp or a similar unit is basically a heating apparatus relying on radiation heat. Therefore, it takes a certain time to heat the fixing roller to a predetermined temperature suitable for fixation after the power source is turned on (hereinafter called "warm up time") no matter how high the thermal radiation of the fixing roller is. On the other hand, in order to increase the value of a product such as a copying machine, it is considered important to make the equipment more compact, as well as to make the fixing device more energy efficient (lower power consumption) and to make the user's operation easier (quicker print).
In case of the conventional fixing device described above, there is a limit in the electric power to be supplied to the radiant heat source due to the constraints arising from the equipment specifications. The only way, therefore, to shorten the warm up time is to reduce the thermal capacity of the fixing roller. Thus, various schemes have been tried to reduce the thickness of the roller, particularly to make the thickness of the base body as thin as possible.
However, reduction of the thickness of the fixing roller results in the drop of the rigidity of the fixing roller, which in turn causes deformations of the roller under high temperature conditions and deterioration of paper feeding performances. If, on the other hand, a lower pressure is used to avoid such a roller deformation, it causes a deterioration of fixing performance. Because of these problems, there is a limit to the thinning of the fixing roller, so that there has been a limitation to the shortening of the warm up time by means of the thinning of the fixing roller.